The development of powder-metallurgical and ceramic construction components and the expansion of areas of application for construction parts so manufactured are closely connected with the capability of obtaining higher compression densities. The results are higher densities and higher mechanical strength values for the sintered parts manufactured from such powders. For this reason, in static powder compression operations, the pressing pressures currently applied reach the limits of the tool steels and other tool materials used. However, it is known that it is not possible to obtain by static compression compacts with a density that is approximately free of pores. For example, the maximally obtainable pressing density of iron and steel powders comes to approximately 90% of the theoretical density, because in the course of the pressing process, the powder particles increasingly support each other in the closed die, so that the pressing pressure required for the further compression increases asymptotically, which means the practical compression limits can be reached quickly.
Possible processes for further increasing the density of compressed powders include, for example, the hot pressing method, where high compression and sintering is obtained via the plastic flow of the powder. For example EP 0 375 469 describes such a process under the title "Process For The Electric Compression Of A Preshaped Workpiece Present In The Form Of Particles". In said process, workpieces are pre-compressed in the pressing die and subsequently compressed further and sintered by feeding an electric direct current directly into the suitably insulated pressing punches.
Other processes can be classified under the term "dynamic compression". In such processes, the pressing forces are applied in spots or lines, such forces increasing in time. Mainly with compacts of a more complex shape, such processes cause inhomogeneities in the material, and cracking to a nonnegligible extent.
A special group of processes for the compression of powder can be classified under the terms "ultrasound" and "explosion" processes. The ultrasound processes permit a denser packing position of individual powder grains only during the initial stage of the powder compression. It is not possible in practical application to manufacture compacts that are genuinely dense. A combination of compression by means of sound waves and sintering is described in DE-OS 31 38 223. The explosion processes are difficult to handle in practical life. According to such processes it is possible only to manufacture from powder compact shapes of construction components. More frequently, comparatively thin layers of powder are pressed onto compact molded bodies, using the explosion process. Such processes are not suitable for application in the production of mass-produced components, even though powder compacts having up to almost the theoretical density are obtained by explosion compression. Such a process is described, for example in DE-OS 27 38 674.
As part of the further development of the aforementioned hot-pressing process, electrically conductive powder materials are currently first prepressed statically, and subsequently compressed further and, at the same time, sintered in the pressing mold by means of electric current pulses. Highly varying implementations of the process are possible depending on the practically used pulse conditions. One variation represents a direct-sintering process with pulsed passage of current through the part to be produced over long periods of time. Such a process is described in DE-OS 27 55 855, whereby strong electric current pulses are inductively generated in the pressed material. In connection with the process variation that has become known under the name "plasma sintering", the pulse conditions created are such that in the prepressed powders with spacings remaining between the individual powder grains, an electric discharge plasma is produced, i.e., particles that are ionized at high temperatures. Such particles lead to a local melt-down of the powder particles, to plastic flow, and thus to dense sintering. In this process, the important secondary results are that oxide skins and surface impurities of the powder grains are broken down, which permits the sintering of powder grains that are otherwise difficult to sinter. Said process has been described in greater detail, for example in the scientific paper by M. Ishiyama under the title "Plasma Activated Sintering (PAS) System", published in the Proceedings of 1993, Powder Metallurgy World Congress in Osaka, Japan. A comparable process is described, for example in EP 0 451 969 under the title "Sintered Composite and Process for its Manufacture".
Each of said processes involves high-temperature sintering of molded components in the pressing mold, and the sintering operation is admittedly, substantially reduced as compared to conventional methods of sintering pre-pressed molded parts; however, the sintered articles to be produced have to remain for about 5 to 10 minutes in the equipment - which consists of the pressing mold and the pulse generator - at the sintering temperatures usually applied, which means that such methods are not suitable for manufacturing sintered, mass-produced components at low cost.
The process applied in EP 0 451 969 is illustrated in FIG. 3 of this patent, and described in greater detail in column 3 of the specification of this patent. A pressing die consists of a female pressing die having an electrically insulating coating on the inside. The female die forms the lateral wall of the pressing die. The powder charged in the female die is compressed under the customary conditions by the axial stroke of a bottom and a top metallic pressing punch. Said top and bottom punches are connected to the capacitor of a pulse discharging system. By closing a short-circuit switch, a high voltage is applied between the top and bottom punches, which permits an electric discharge between the punches to pass through the pressed material. Such discharge is repeated many times for compressing and simultaneously sintering the material. The pressed material is slightly melted and sintered together by the heat generated in the course of such repeated discharging.